The invention relates to aluminum electrical terminal connectors, including connector lugs having a threaded opening for receiving a clamping screw which is tightened down to hold an electrical terminal in the lug.
The use of aluminum in electrical distribution systems as an electrical conductor is known. Aluminum is lighter and more economical than copper. However, aluminum has certain disadvantages which have hindered its total acceptance as a substitute for copper. One of these disadvantages is the necessity to plate the aluminum to prevent the formation of a dielectric oxide layer thereon. Another disadvantage is that in-service temperature cycling due to on/off electrical current flow causes changes in the physical size of the aluminum conductor as a result of thermal expansion and contraction. This phenomena causes joined parts to creep, thereby loosening clamped connections and creating increased voltage drops at the respective connections, which in turn may cause potentially dangerous elevated temperatures at such connections.
The above noted problems are particularly manifested in aluminum connector lugs, especially when such lugs are utilized to connect aluminum wire conductors. The lug includes a body having a main opening for receiving one or more electrical terminals, and a transverse threaded opening for receiving a clamping screw which is tightened down to engage and frictionally hold the terminals in the lug body. Sufficient clamping pressure should be applied to penetrate any oxide layer formed on the terminal. The plating for the connector lug should be durable enough to withstand such clamping pressure.
If the plating on both the lug and the terminal breaks down, an aluminum to aluminum connection occurs. While this initially provides a connection with low electrical resistance, thermal expansion may create relative movement to expose bare aluminum, which exposed surface will be subject to oxidation, which increases the voltage drop at the connection.
It has been found that a problem associated with plating of aluminum electrical connector lugs is uniformity of the coating along the threads in the threaded opening. Prior plating methods do not provide uniform plating thickness at the roots and crowns of the thread. The plating at the crowns is substantially thicker than the plating at the roots. This provides a condition which adversely affects the mechanical cooperation of the threads of the screw with the threads in the threaded opening of the lug body.
Another problem with nonuniform thickness of the plating layer along the lug threads is the adverse affect on electrical current distribution. The screw typically carries as much as 15% of the load current. It is desired to at least maintain this load current and resultant heat distribution. A nonuniform coating layer provides varying electrical resistance paths and hence may diminish the load current share through the screw, which is undesirable.
It is believed that prior attempts to satisfactorily electroplate an aluminum connector lug failed for various reasons. The aluminum connector lug is typically provided with a deoxidation pretreatment protective layer, such as a zinc or tin film, for example by a zincate process. The connector was then nickel plated. It is believed that in some instances the nickel plating bath was too acidic, e.g. 3 or 4 pH, and rapidly dissolved the zincate layer and then attacked the aluminum surface before the nickel could be electrolytically plated thereon, which exaggerated the problems already inherent in electroplating low current density areas such as the roots.
In the present invention, it has been found that acetate buffered nickel glycolate electroplating provides a solution to the above noted problem. This type of electroplating is known in the art for a strike application for capstan drive motors, "Stable Strike for Plating on Aluminum", Missel et al, Metal Finishing, Aug. 1981, pages 37-42. In the present invention, it has been found that instead of using the nickel glycolate as a strike, such process and bath may also be used to build up layer thickness in aluminum connector lugs, and it has been found that the coated layer has uniform thickness at the roots and crowns of the lug threaded opening
It is believed that the acetate buffered nickel glycolate successfully provides uniform thickness plating at the roots and crowns because of reduced acidity of the nickel electrolyte reacting more slowly with the zincate layer. It is also believed that such bath provides low cathodic efficiency in high current density regions, also slowing the process, and providing a more uniformly thick plating at the roots and crowns. The crowns are high current density regions and the roots are low current density regions during electroplating. It is also believed that a low cathodic efficiency near neutral nickel electrolyte dissolves the zincate layer at a much slower or negligible rate and permits immediate deposition of nickel on the zincate layer without the aluminum being attacked by the otherwise highly acidic electrolyte.